This application seeks funding to expand and improve the North Atlantic Population Project (NAPP) database. NAPP is a unique collaboration of eight leading data producers who have leveraged resources to create an extraordinary cross-national historical database. Over the past four years, the collaborating partners have cleaned, edited, coded, harmonized, and disseminated almost 90 million records describing basic demographic characteristics of the populations of five countries. These data include the entire population of Britain and Canada in 1881, Iceland in 1870, 1880, and 1901, Norway in 1865 and 1900, and the United States in 1880. These are the only complete-count national microdata available for scholarly research, and they represent an extraordinary resource for the study of small areas and population subgroups. This fundamental social science infrastructure is already stimulating broad-based comparative investigations of economic development and demographic change. To exploit the research potential of these data, we now propose: (1) expanding the chronological and geographic dimension of the database by incorporating data from additional census years for each country and adding data from Sweden;(2) coordinating national projects to link individuals between censuses, which will permit longitudinal analysis;and (3) improving NAPP variables, data editing, documentation, and web- based dissemination tools. The availability of multiple cross-sections for the population of the North Atlantic world in the nineteenth and early twentieth centuries will open up vast new terrain in the fields of history, economics, demography, and sociology. In addition, linked samples hold the promise of finally resolving some of the longest-running debates in social and economic history. Scholars will be able to gauge trends and differentials of social and geographic mobility and the interrelationship of geographic and economic movement far more reliably than heretofore. The expanded NAPP database is directly relevant to the central mission of the NIH as the steward of medical and behavioral research for the nation: NAPP will advance fundamental knowledge about the nature and behavior of human population dynamics. This basic infrastructure will advance health-related research on population growth and movement, fertility, mortality, and disability.